Methods and measuring systems of the above kind are utilized for identifying harmful substances in ambient air. The detection of harmful substances after accidents or catastrophes is important, for example, for emergency personnel of fire and police departments. A list of thirty-three of the most frequently encountered harmful substances has been prepared for this reason in Germany for the protection of emergency personnel. This list includes also concentrations, the so-called emergency tolerance values (ETV list), at which one can assume that it is possible to safely work without breathing protection over a time period of four hours. At the present time, work is ongoing on an expansion of the list while considering the international acute exposure guide line levels (AEGL).
For military personnel and most recently also for civilian emergency personnel, especially the additional detection of chemical warfare substances or explosives is of interest.
These harmful substances can be detected partially with measuring systems which primarily comprise individual gas detectors or combinations of different gas detectors. The measurement signals of the individual gas detectors can then be compared to signals measured in advance or stored and the measured state can be described. As detectors, the following are, for example, appropriate: photo-ionization detectors (PID), electrochemical cells (EC) and metal-oxide sensors (MOS). Measuring apparatus, which supply two-dimensional data, that is, spectra, are also used. Examples of these are mass spectrometers (MS), Fourier transform infrared spectrometers (FTIR) or ion mobility spectrometers (IMS).
Simple detectors such as a PID, MOS or EC are suitable for the detection of many harmful substances with their detecting limits in the upper ppb range or lower ppm range. However, these detectors are too insensitive for detecting warfare materials. Furthermore, their selectivity is often insufficient in order to detect harmful substances with the necessary reliability.
U.S. Pat. No. 2,959,677 discloses the essential functional features of a PID. With the aid of a UV lamp, the gas to be detected is ionized and thereafter is electrically detected. What is primarily significant is the ionization potential of the compound to be detected. In the event that the energy of the UV-radiation is greater than the ionization energy of the compound, then these energies can be detected. What is disadvantageous here is that many harmful substances cannot be detected. There is no spectral information supplied. Furthermore, it is disadvantageous that PID-lamps are rapidly contaminated which leads to poorer signal exploitation.
U.S. Pat. No. 3,631,436 discloses the essential functional features of the metal oxide sensors. These sensors react with reducing and oxidizing gases. These sensors have a relatively intense cross sensitivity and cannot be used for detecting individual substances or as warning devices because of the high rate of false alarms. MOS sensors are characterized by very rapid response times after an exposure to gas; however, the sensors have the disadvantage that the decay times are significantly longer.
Electrochemical cells are more selective than MOS sensors. A determination of individual substances is nonetheless not possible with the detectors because, here too, cross sensitivities occur, that is, there are no electrochemical cells available for all substances. The essential functional features of the electrochemical cells are disclosed in U.S. Pat. No. 3,925,183.
The ion mobility spectrometers (IMS) or the plasma chromatograph are known for some time. In contrast to other spectrometers, no moveable or complex individual parts are needed in the IMS so that these systems can be built small and cost effectively. For many compounds, very low detecting limits in the ppt-ppb range can be achieved. For this reason, these systems have been utilized for years by the military to detect warfare substances. A description of the individual components in an IMS can be found, for example, in U.S. Pat. No. 3,621,240. The different mobility of ions is utilized by the IMS. These apparatus comprise an inlet system, an ion source, an electrical drift tube and a measurement sensor for detecting low electrical currents which are generated by the impinging ions. For the ion sources, radioactive Ni63 foils are typically used and, in the electrical drift tube, the ions are separated in accordance with their mobility at ambient pressure after a defined start by means of an electrically switched grating or grid. Primarily, air molecules are ionized in the ion source at atmospheric pressure. These air molecules thereafter ionize water clusters which are also referred to as reactant ions. Thereafter, the harmful substances are ionized via proton transfer reactions, electron transfer reactions or proton abstraction reactions. By changing the polarity of the drift path, positive ions can be detected in the positive operating mode or negative ions in the negative operating mode.
In mobile systems, the inlet is, as a rule, a membrane. A membrane inlet system for an IMS is described in U.S. Pat. No. 4,311,669. It is advantageous that, because of the membrane, the influences on the measuring signal via disturbance quantities are reduced which include, for example, moisture, pressure and temperature and, for this reason, IMS systems can be manufactured to be small and portable. It is disadvantageous that the membrane causes the measuring system to react with more inertia with respect to its response time.
What is especially disadvantageous in the IMS, is the long time duration which must be allowed to elapse until the system is again operationally ready for measurement after switching on the apparatus. The IM detector requires this time because the IM detector must flush disturbing substances out of the system which have accumulated during the switched-off state. It is furthermore disadvantageous that, for short-term overdosing, the system is no longer measurement ready and must be flushed for several minutes up to hours. It is also disturbing that the spectra are dependent on concentration.
A further problem is the in part low selectivity of the IMS. One reason is that, often, the harmful substances of interest are not ionized because of competing reactions in the ionization chamber and therefore cannot be detected. These competing reactions can lead to the situation that many harmful substances having lower proton affinities, such as many solvents, do not even appear in the spectrum, for example, in the presence of gases such as ammonia. On the other hand, the detection of warfare substances can be made more difficult or even impossible by the presence of solvents in high concentrations (ppm). The false alarm rate is then increased by the superposed spectra in a mixture of gases. Furthermore, warfare substances having low proton affinity or electron affinity are not determined with the required detection limits.
A further disadvantage of the IMS is the limited measuring range which, for example, for a beta radiator as ionization source, amounts to typically maximally two orders of magnitude. A quantitative statement is therefore difficult.
It is furthermore problematic that many harmful substances exhibit a low vapor pressure so that the detection limits of the detectors are not adequate in order to register these limits.
A publication of L. V. Haley entitled “Development of an Explosives Detection System using fast GC-IMS Technology” (Proceedings 32 Annual 1998 International Canada Conference, Alexandria, Va. USA (1998), pages 59 to 64) describes the combination of a gas chromatographic device with an ion mobility spectrometer (IMS).
U.S. Pat. No. 4,987,767 discloses a detection system for explosive gases. A vapor and/or a particle emission is conducted in a test chamber wherein a separation of the vapor mixture and/or particle emission takes place which is then supplied to a gas chromatographic device and an ion mobility spectrometer (IMS) for detailed substance analysis.
In the last two mentioned publications, it is disadvantageous that the measuring method is very complex and expensive because of the use of a gas chromatographic device.
Furthermore, a method is known from a publication by O. D. Sparkman (The Twelfth Sanibel Conference on Mass Spectrometry: Field-Portable and Miniature Mass Spectrometry; J. Am. Soc. Mass Spectrom. 11 (2000), pages 468 to 471) wherein a combination of an IMS and additional detectors is used to determine gaseous harmful substances. Here, with the additional detectors, the harmful substances are detected which are not detectable with the IMS because of their low proton affinities or low electron activity. However, in this publication, there is no suggestion as to in what manner the IMS is coupled to the additional detectors. Furthermore, there are no measures described which prevent the measuring range of the IMS to be exceeded or that there be a drop therebelow during the measurement.